Eye width measurement and margining in communication systems

ABSTRACT

Generally, this disclosure describes eye width measurement and margining in communication systems. An apparatus may be configured to: decouple a phase detector from a CDR loop filter of a receiver under test in response to synchronizing a margining clock signal to a receiver clock signal; apply a margining input to the loop filter, the margining input configured to shift a frequency of the margining clock signal by a constant amount related to the margining input; compare a first bit stream and a second bit stream and configured to detect an error, the first bit stream related to a transmitted bit stream; and count cycles of the receiver clock signal or the margining clock signal, wherein an eye width associated with the receiver under test is related to the margining input, the frequency of the receiver clock signal and a count of clock cycles when the error is detected.

REFERENCE TO PRIORITY APPLICATION

This present application is a Continuation of U.S. patent applicationSer. No. 14/968,616 filed Dec. 14, 2015, which is a Continuation of U.S.patent application Ser. No. 14/667,146, filed Mar. 24, 2015, now U.S.Pat. No. 9,215,061, which is a Continuation of U.S. patent applicationSer. No. 13/836,383 filed Mar. 15, 2013, now U.S. Pat. No. 8,989,329,all of which are incorporated herein by reference in their entirety.

FIELD

This disclosure relates to eye width measurement, more particularly eyewidth measurement and margining in communication systems.

BACKGROUND

Today's SoCs (System on Chip) and processor products for client, serverand mobile applications rely on high speed data communication links inorder to communicate large amounts of data across the motherboard orcomputer/server rack backplane between data sources and destinations.Serial IO (input/output) links (e.g., SERDES, Serializer-Deserializersystems) are among the most common types of high speed IOs in use today,e.g., where a 32 bit (or more) bus is being serialized into a singlehigh speed physical wire-line (e.g., wired) and/or wireless channel.There are many types and families of serial IOs, including PCIe(Peripheral Component Interconnect Express), SATA (Serial ATA), KTI, QPI(QuickPath Interconnect) and wire-line Ethernet links, each withcorresponding benefits and characteristics. Links with symbol rates of5, 8 or 10 GB/sec per physical channel are in relatively wide use, andeven higher rates may be possible and are contemplated for commercialuse.

In high speed serial data communication links employing NRZ (non-returnto zero), PSK (phase shift keying) and PAM (pulse amplitude modulation)modulations, Bit Error Rate (BER), Time-to-Link (TTL) and eyemeasurements are relatively common metrics for evaluating linkperformance. A link specification typically includes maximum permissiblevalues for BER and TTL. In order to qualify a link, it must have BER andTTL that are less than these maximums. When designing serial links forhigh volume systems, merely meeting these criteria is not enough toensure robust performance over the entire ensemble of scenarios andtolerances that may be experienced in the field. Rather, good designpractices suggest that the serial link design include additionaloperational margins that will ensure performance in degraded conditions.Such margins and their measurement are a central part of a productproduction quality process.

Link characterization by margining the information bearing signal eyepattern at the receiver decision junction (e.g., at an input to dataslicer(s) that are configured to determine whether a signal correspondsto a logic zero or a logic one) is a generally accepted marginingmethod. An eye pattern specification may generally include an eye maskthat provides eye opening minimum requirements in order to meet adesired link BER requirement. However, in order to provide a more robustdesign, communication systems are typically designed to obtain a moredesirable performance margin by expanding an area around the eye maskand using the expanded area as the minimum eye opening target.

In order to qualify a link, test data is typically required showing thatthe link BER is below the specified maximum BER, the time-to-link isbelow the maximum time-to-link and the eye opening does not impinge onthe eye mask. In a relatively low speed SERDES circuit, eye patternmeasurements are typically performed very close to an input to a dataslicer that makes the decision whether a received bit is a zero or aone. However, in high speed links, accessing this point with externaltest equipment is generally difficult and may degrade performance.Further, test equipment access is usually limited to the Receiver inputterminals (which are not where the actual eye after processing by thereceiver may be measured). Thus, high speed data links provide aparticular challenge for testing.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of embodiments of the claimed subject matterwill become apparent as the following Detailed Description proceeds, andupon reference to the Drawings, wherein like numerals depict like parts,and in which:

FIG. 1 illustrates a communication system consistent with variousembodiments of the present disclosure;

FIG. 2 is an example of an eye pattern that may be used to characterizea communication system;

FIG. 3 illustrates eye measurement circuitry, a data slicer module andan embodiment of a clock and data recovery CDR module consistent withthe present disclosure;

FIGS. 4A and 4B illustrate examples of waveforms generated during eyemargin characterizations performed using a method consistent with thepresent disclosure;

FIG. 5A illustrates one example of an eye measurement system configuredto characterize the eye margin consistent with the present disclosure;

FIG. 5B illustrates another example of an eye measurement systemconfigured to characterize the eye margin consistent with the presentdisclosure;

FIG. 6 illustrates another example of an eye measurement systemconfigured to characterize the eye margin consistent with the presentdisclosure;

FIG. 7 illustrates another embodiment of a system for eye widthmeasurements and margining consistent with the present disclosure; and

FIG. 8 illustrates a flowchart of exemplary operations consistent withan embodiment of the present disclosure.

Although the following Detailed Description will proceed with referencebeing made to illustrative embodiments, many alternatives,modifications, and variations thereof will be apparent to those skilledin the art.

DETAILED DESCRIPTION

Generally, this disclosure describes an apparatus, system and method formeasurement, characterization and margining an eye pattern for arelatively high speed communication system. In particular, an eye-width(e.g., horizontal dimension of the eye pattern) measuring and marginingtopology is described that may be used for receiver clock and datarecovery (CDR) circuits that employ voltage controlled oscillators(VCOs) and/or digitally controlled oscillators (DCOs). An eye patternmay be characterized using a known pseudorandom bit sequence (PRBS)and/or an unknown bit sequence (e.g., actual data). The system andmethod may be further applied to phase interpolation (PI) based CDRcircuits. The method may be implemented on-die, using circuitryavailable on-die. In one example, the method is configured to provide aresolution on the order of 0.5 picosecond for a 10 GB/sec data signal.

Using an existing PI-based CDR topology, an eye measurement may beperformed as follows. A receiver is allowed to adapt and stabilize, andafter a particular bit-error-rate (BER) has been achieved, the CDRcircuit is precisely locked on a correct data slicer sampling instance(“ideal” sampling instance). Lock may be determined based on receivedsignals and/or lock may be assumed after a sufficient time period haselapsed configured to allow initialization transients to subside and thesystem to achieve steady state. All equalizers and amplifiercoefficients are then fixed at their current values. A pseudorandom bitsequence (PRBS) may then be transmitted from the transmitter to thereceiver. The receiver, knowing the bit sequence a priori, may thenrelatively easily detect errors in the received bit stream. A phaseshift may then be added to the sampling instance so that the data slicernow samples at a horizontal displacement from the ideal samplinginstance. Sampling continues until an error is detected, at which pointthe phase shift is recorded and that test round ends. This process isrepeated many times in order to accumulate data for averaging so as toimprove the statistical properties of the result. The phase shift maythen be increased by an incremental amount and the process repeated.This PI-based procedure scans the eye to the right of the ideal samplinginstance. The PI-based procedure may be repeated while subtracting phaseshifts in order to scan the eye to the left of the ideal samplinginstance, thus scanning the full eye width.

There are several limitations and drawbacks of this PI-based procedurefor scanning the eye width using PI-based CDR circuits. Itsimplementation is limited to PI-based CDRs and may not be adaptable tovoltage controlled oscillator (VCO) based CDR circuitry. The resolutionassociated with the described PI-based procedure is limited by adifferential nonlinearity associated with the PI (typically on the orderof 2.5 to 5.0 picoseconds, in current technology). In relatively highdata rate communication systems (e.g., greater than or equal to 10GB/sec), such resolution is a serious limiting factor that typicallyrequires circuit level optimization and tuning to achieve. The PI-basedprocedure may not be suitable for fully digital CDR architectures. For acommunication system, e.g., SERDES, that includes a decision feedbackequalizer (DFE), the PI-based procedure may be utilized only until afirst error is detected unless a unique slicer is used for eye-widthmeasurements and a unique slicer is used for data detection. If a singleslicer is used, then when an error is detected, DFE error multiplication(due to the feedback) may cause a transient eye closure and possibly aburst of errors that will require halting the test procedure andre-adaptation of the CDR circuitry and the channel before the test maycontinue. This may increase the time to perform the test and may distortits result. The phase interpolator used for this PI-based proceduretypically requires a linear phase interpolator that has to be calibratedin order to quantify the test results. Such calibration can be a complex(e.g., time consuming) and relatively expensive procedure. The PI-basedprocedure cannot be implemented entirely on-die. Rather, relativelyexpensive external test equipment is generally used to calibrate andprocess measurement results.

One benefit of the PI-based procedure is that it is configured tocharacterize the eye width while the CDR operates closed loop. API-based CDR typically includes two control loops that operateindependently. A first control loop is a frequency acquisition andtracking loop that may be analog or digital and a second control loop isa phase acquisition (e.g., symbol timing) and tracking loop that alsomay be analog or digital. Since the second loop is configured to tracksymbol (e.g., bit) timing directly, a timing shift (e.g., phase shift)may be applied relatively easily through selection of a different PIphase in each UI (symbol/bit unit interval). For a VCO-based CDR, thephase may not be programmed directly since there is only one controlloop that controls the frequency. As a result of frequency modification,the phase may also be shifted. Moreover, while operating in closed loop,any phase increment/decrement (that might be applied to the CDR throughfrequency offset) may result in phase correction of the control loop.

A system and method consistent with the present disclosure areconfigured to address at least some of the described short-comings ofexisting PI-based procedures. The system and method may be utilized inVCO-based CDR architectures as well as PI-based CDR architectures withanalog or digital or mixed implementations and is configured to operateat data rates, including, but not limited to, 10 GB/sec and higher.

FIG. 1 illustrates a communication system 100 that includes atransmitter (TX) 102, a channel 104 and a receiver (RX) 106. Thetransmitter 102 is configured to receive serial data in (“bit streamin”), configure the serial input data (e.g., combine the data with aclock signal) for transmission and transmit the clock and data downchannel 104 to receiver 106. Receiver 106 is configured to receive andprocess the received clock and data signal and to provide the serialdata out (“bit stream out”). During transmission the serial data inputmay be distorted by, among other things, bandwidth limitations of thechannel 104, attenuation associated with the channel 104, noise, etc.The receiver 106 is configured to process the received clock and datasignal to recover the bit stream in and to provide as output acorresponding bit stream out.

The receiver 106 may include a receiver digital signal processing moduleRxDSP, a microprocessor and/or logic circuits 108 configured to performdigital signal processing activities for the receiver 106, an equalizer110, a variable gain amplifier VGA 112, decision feedback equalizer DFE114, a data slicer module 116 and a clock and data recovery module (CDR)118. The equalizer 110 is configured to amplify higher frequencycomponents of the received signal to compensate for the bandwidthlimitations of the channel and the VGA 112 is configured to amplify theequalized signal. The DFE 114 is configured to perform furtherequalization to reduce inter-symbol interference using, e.g., feedbackfrom the data slicer module 116.

Data slicer module 116 may include one or more data slicers that areconfigured to discretize and sample the received, amplified andequalized signal from the DFE 114. During normal operation, the dataslicer module 116 is further configured to receive a recovered clocksignal from CDR 118 that synchronizes the sampling and discretizing witha clock extracted from the received signal. The CDR 118 receives both areceiver bit stream from the data slicer module 116 as well as an erroroutput signal configured to indicate received signals outside areference voltage range (±Vref). Receiver 106 relies on CDR 118 forclock and data recovery in order to recover the bit stream in from thereceived signal that has been attenuated and filtered by the channel104.

FIG. 2 is an example of an eye pattern 200 that may be used tocharacterize a communication system. The vertical axis corresponds toamplitude and the horizontal axis corresponds to time. An ideal samplinginstance may be defined as a point in a unit interval where a signal tonoise ratio is maximum and may correspond to the vertical axis in somecases. An eye pattern may be generated by a sequence of bits (bit streamthat may include clock and data information) and provides an indicationof margin associated with the communication system. A critical eyeopening 202 may be specified in order to achieve a desired (and/orrequired) BER. For design, a larger opening, e.g., region 204 may bespecified in order to ensure a margin and a greater likelihood that thecommunication system may continue to operate under less than optimalconditions. In order to qualify a communication system, generating aneye diagram may be desirable. In particular, determining an eye width203 may provide an indication of sensitivity to timing errors.

FIG. 3 illustrates eye measurement circuitry 300, a data slicer module302 and an embodiment of a clock and data recovery CDR module 304consistent with the present disclosure. The data slicer module 302 is anexample corresponding to data slicer 116 of FIG. 1 and the CDR 304 is anexample corresponding to CDR 118 of FIG. 1. The data slicer module 302may include monitor slicer 306. The data slicer module 302 includes apositive error slicer 308, a data slicer 310 and a negative error slicer312. The slicers 306, 308, 310, 312 are configured to discretize andsample the amplified and equalized signal received from, e.g., DFE 114.The slicers 306, 308, 310 are configured to sample at a recovered clockfrequency provided by CDR 304 and to output a logic high (or low) whenthe sampled input signal is above (or below) a reference input value.The negative reference error slicer 312 is configured to sample at therecovered clock frequency and to output a logic high (or low) when thesampled input signal is below (or above) the negative error referenceinput value. The output of the data slicers 306, 308, 310, 312 isprovided to CDR 304.

CDR 304 may include phase detector and symbol timing module 320, a loopfilter 322, an oscillator 324 and coupling circuitry 326. During normaloperation, phase detector and symbol timing module 320 is configured toreceive output signals from data slicer 310 (“receiver bit stream”) andthe error slicers 308, 312, to perform phase detection and symbol timingfunctions on the received signals. Phase detector and symbol timingmodule is configured to provide a corresponding output to loop filter322 via coupling circuitry 326. The loop filter is configured to adjusta frequency of oscillator 324 so that a clock signal may be recoveredand “lock” obtained on the received bit stream. Coupling circuitry 326may include, but is not limited to, a switch (analog or digital), and/ora register configured to store digital information. Coupling circuitry326 is one example of circuitry that may be included on-die configuredfor design-for-test and/or may be included for general operation of CDR304.

For example, phase detector and symbol timing module 320 may beconfigured to implement a phase detector and symbol timing method suchas Qureshi that is configured to utilize three consecutive samples ofdata (e.g., from data slicer 310) and error (e.g., from error slicers308 and 312) and to adjust an output of the phase detector and symboltiming module 320 and ultimately the input to the oscillator 324 based,at least in part, on the three consecutive samples. In another example,phase detector and symbol timing module 320 may be configured toimplement another detector and symbol timing module such as Mueller andMuller. Mueller and Muller is a one-sample-per symbol decision-directedmethod where “decision-directed” corresponds to the fact that in theMueller and Muller method, the phase detector and symbol timing module320 may be configured to estimate a bit in the transmitted bit stream inand typically does so based on a current sample, e.g., an output of dataslicer 310. Other phase detection and symbol timing methods may beutilized and it may be appreciated that operation of system and methodconsistent with the present disclosure is independent of the specificphase detection and symbol timing module used.

Loop filter 322 is configured to low pass filter (e.g., average) thesignal received from the phase detector and symbol timing module 320.For example, loop filter 322 may include a proportional stage 330 and anintegrator stage 332, as described herein. The filtered signal may thenbe provided to oscillator 324 and is configured to adjust an outputfrequency of the oscillator 324 where the output of oscillator 324corresponds to a recovered clock signal provided to data slicer module302. When locked, the recovered clock signal corresponds to a receiverclock signal associated with the receiver bit stream output of the dataslicer module 302. In some embodiments, oscillator 324 may be a voltagecontrolled oscillator (VCO). In some embodiments, oscillator 324 may bea digital controlled oscillator (DCO). The output of oscillator 324 isfed back to the data slicer module 302 and is configured to synchronizethe recovered clock and data signals to the receiver clock signal. Theoutput of data slicer 310 may then correspond to bit stream out in FIG.1 and the outputs of the error slicers 308, 312 may the correspond toerror out in FIG. 1.

Eye measurement circuitry 300 includes a controller module 340, amargining input module 342, a compare module 344 and a counter logicmodule 346. Eye measurement circuitry 300 may include a PRBS(pseudo-random bit sequence) generator 348. The margining input module342, the compare module 344, the counter logic module 346 and the PRBSgenerator 348 may be included and/or may exist on-die independent ofwhether eye measurement and margining is implemented as describedherein. Although illustrated as individual modules, one or more of themodules may be combined into a single module, consistent with thepresent disclosure.

The controller module 340 is configured to manage eye width measurementand margining. The controller module 340 may be configured to providemeasurement data to, e.g., a computing device including a userinterface, so that tests can be initiated and results communicated. Thecontroller module 340 is configured to provide commands to margininginput module 342, counter logic module 346, PRBS generator 348 (ifactive) and coupling circuitry 326. The controller module 340 may beconfigured to receive an indication that a BER has been met in order toinitiate an eye width measurement process as described herein. In someembodiments, controller module 340 may be included on-die. In someembodiments, controller module 340 may be off-die and coupled to on-dieelements of the eye measurement circuitry 300 and/or CDR 304 forperforming eye margining, as described herein.

Depending on a test configuration, PRBS generator 348 may or may not beutilized for eye width measurement. When PRBS generator 348 is utilized,controller module 340 may be configured to provide test sequenceselection data to PRBS generator 348 so that it may generate the correctpseudo-random bit sequence for the eye width measurement process. One ofa plurality of PRBS sequences may be utilized for testing purposes. Suchsequences may be utilized for determining BER as well as eye widthmeasurement and margining consistent with the present disclosure.Providing test sequence selection data is configured to allow the PRBSgenerator 348 to generate a same PRBS sequence as was transmitted, e.g.,bit stream in to transmitter Tx 102 in FIG. 1.

Controller module 340 may be configured to provide margining inputselection data to margining input module 342. For example, as describedherein, margining input may be in the range of about 1000 PPM to about10,000 PPM. Controller module 340 may thus be configured to select aspecific margining input, e.g., 5000 PPM. The margining input may beconstant for at least a portion of a test sequence (e.g., eye widthmeasurement). For example, the specific margining input may be set by auser communicating with controller module 340 and may correspond to adesired resolution of the eye width measurements as described herein. Insome embodiments, the desired resolution may vary based, at least inpart, on a distance from the edge of the eye, as described herein. Inanother example, controller module 340 may be configured to select thespecific margining input based, at least in part, on a bit rate of bitstream in, e.g., 10 Gb/sec, and/or a desired resolution of the eye widthmeasurement.

Controller module 340 may be configured to adjust Vmon, the referenceinput for monitor slicer 306. In some embodiments, the monitor slicer306 may be configured to facilitate eye width measurements at zero aswell as amplitudes other than zero by selecting Vmon. For example, Vmonmay be adjusted from zero to ±(full eye height+a margin). Thus,controller module 340 may be configured to adjust Vmon in order to setthe amplitude at which eye width may be determined, as described herein.

Controller module 340 may be configured to receive an indication that adesired BER has been achieved. Controller module 340 may be configuredto initiate an eye width measurement based, at least in part, on thisindication. A test bit stream in (e.g., PRBS) may be provided as inputto transmitter 102 and CDR 304 may be allowed to synchronize recoveredclock signal to the receiver clock signal. As used herein, during eyewidth measurement and/or eye characterization, the clock signal outputfrom the oscillator 324 is termed a margining clock signal. Thus, themargining clock signal may be synchronized to the receiver clock signalwhen the recovered clock signal is synchronized to the receiver clocksignal.

Controller module 340 may be configured to decouple phase detector andsymbol timing module 320 and loop filter 322 via coupling circuitry 326in response to the synchronizing. For example, coupling circuitry 326may be a switch and controller module 340 may be configured to commandcoupling circuitry 326 to open. Controller module 340 may then commandloop filter 322 to set a gain of proportional stage 330 to zero and tofreeze any gains associated with integrator stage 322 at their currentvalues. The current values of the gains associated with the integratorstage 322 correspond to oscillator 324 current output frequency (e.g.,margining clock frequency). Thus, since the recovered clock signal andthe receiver clock signal are synchronized when the phase detector andsymbol timing module 320 and loop filter 322 are decoupled, the currentintegrator values correspond to recovered clock signal synchronized toreceiver clock signal.

Controller module 340 may then be configured to trigger margining inputmodule 342 to apply the margining input to loop filter 322. Controllermodule 340 may be further configured to reset counter logic module 346and to enable counter logic module 346 to start counting. The effect ofthe added margining input is to cause a frequency difference between theoutput of oscillator 324 (e.g., margining clock) and the received signalclock. Because of the frequency difference, a phase difference begins toaccumulate. This phase difference increases by one phase differenceincrement with each clock cycle (e.g., unit interval, symbol interval).By counting the symbol intervals beginning with the initial applicationof the phase difference until an error is detected, a portion of the eyewidth from the center to the right edge, e.g., portion 206 may bedetermined. Closing the loop and allowing the system to stabilize (e.g.,allowing the margining clock signal to synchronize to the receiver clocksignal), followed by applying a negative margining input and repeatingthe process may allow a portion of the eye width from the center to theleft edge, e.g., portion 208 to be determined. The process may berepeated multiple times in order to improve statistical characteristics.The process may be similarly repeated with adjustments to Vmon betweentest sequences in order to characterize the eye pattern (e.g., aboveand/or below zero).

In some embodiments, controller 340 may be configured to measure anentire eye width (rather than one half of the eye width) in one scan.For example, controller 340 may be configured to trigger margining inputmodule 342 to apply the margining input to loop filter 322. Controller340 may then be configured to monitor compare module 344 until an erroris detected. Controller 340 may then be configured to reset counterlogic module 346, to enable counter logic module 346 to start countingand to invert the margining input applied by margining input module 342to loop filter 322. The count when an error is detected may thencorrespond to the entire eye width, e.g., count*ΔT. Thus, the entire eyewidth may be determined in one scan.

The margining input may have units of PPM (parts per million) typicallyused to quantify clock accuracy. In some embodiments, the margininginput may be in the range of about 1000 PPM to about 10,000 PPM. Themargining input may be selected to be at least one order of magnitudegreater than typical clock accuracy. The phase difference increment maybe determined as follows. Assume the output frequency of oscillator 324is f when the loop is opened between phase detector and symbol timingmodule 320 and loop filter 322. Thus, f corresponds to the recoveredclock signal frequency, the margining clock signal frequency (when theloop is opened prior to applying the margining input) and the receiverclock signal frequency. For a margining input of m PPM, the marginingclock signal frequency may shift by Δf=mPPM*f=m*f*10⁻⁶ yielding amargining clock frequency of f±Δf, with the plus or minus related towhether the eye width is being swept to the left 208 or the right 206.The phase difference increment may be determined as ΔT=(1/f)−(1/(f±Δf))which, after some algebraic manipulation and recognition that m PPM ismuch less than one, yields ΔT=±mPPM/f. Thus, for a 10 Gb/sec bit streamand a margining input of 5000 PPM, the phase difference increment may bedetermined to be 0.5 picoseconds which corresponds to a minimumresolution of the eye width margining for this example. Thus, amargining input may be selected to achieve a desired resolution based ona data rate.

In some embodiments, a plurality of margining inputs may be appliedduring a test sequence. For example, a relatively larger margining inputmay be applied initially configured to provide a relatively larger phasedifference increment away from the edge of the eye (e.g., near the idealsampling instance). A relatively smaller margining input may then beapplied configured to provide a smaller phase difference increment andtherefore finer resolution closer to the edge of the eye width. The testsequence may then be performed relatively more quickly. Thus, in theseembodiments, both coarse and fine resolution may be provided byadjusting the margining input. Determination of eye width may theninclude each of a plurality of counts of clock cycles corresponding to arespective one of the plurality of margining inputs.

Counter logic module 346 is configured to count a number of clock cyclesuntil an error is detected as described herein. In some embodiments,counter logic module 346 may be coupled to PRBS generator 348 and maythus count clock cycles based on the PRBS bit stream. In someembodiments, e.g., where the PRBS generator 348 is not used, counterlogic module 346 may be configured to count clock cycles based on anoutput from data slicer module 302, e.g., data slicer 310 outputcorresponding to the receiver bit stream. For example, counter logicmodule 346 may be configured to start counting in response to a firsttransition detected after being enabled by controller module 340. Insome embodiments, counter logic module 346 is configured to continuecounting until an error is detected by, e.g., compare module 344. Insome embodiments, counter logic module 346 may stop counting in responseto a command from controller module 340, e.g., if an eye widthmeasurement has timed out without an error being detected. In someembodiments, counter logic module 346 may be configured to capture acurrent count when an error is detected and to continue counting, asdescribed herein.

In some embodiments, counter logic module 346 may include circuitryconfigured to compensate for relatively short duration bit sequencesthat do not include a transition. Such circuitry is configured tocompensate a count value for a number of transitions in a bit streamuntil the error occurred.

Compare module 344 is configured to compare two bit streams configuredto detect an error (e.g., a difference between the two bit streams).Compare module 344 is configured to provide an output signal to counterlogic module 346 when an error is detected. Comparison of the two bitstreams is configured to detect when an eye edge has been reached duringan eye measurement as described herein. The specific bit streams scannedmay depend on characteristics of the particular eye width measurementprocess. In the embodiments that include using the PRBS generator 348,the compare module 344 may be configured to compare the PRBS bit streamoutput from the PRBS generator 348 and a bit stream output from monitorslicer 306 (“monitor bit stream”) or a bit stream output from dataslicer 310, e.g., receiver bit stream. For example, the receiver bitstream may be compared to the PRBS bit stream when measuring eye widthat or near zero. In another example, the monitor bit stream may becompared to the PRBS bit stream when measuring eye width at a level setby Vmon configured to characterize the eye at any level, the level setby Vmon. The monitor bit stream may correspond to the receiver bitstream when Vmon is equal to zero. In the embodiments where the PRBSgenerator 348 is not used, the compare module 344 may be configured tocompare the monitor bit stream and the receiver bit stream. In theseembodiments, Vmon of monitor slicer 306 may be adjusted to facilitateeye width measurement, as will be described in more detail below.Selection of the two inputs to compare may be performed by controllermodule 340, e.g., in response to a user command.

Thus, eye measurement circuitry 302 may be configured to implement avariety of eye width measurements including eye diagramcharacterization. The measurements may be performed by circuitry thattypically exists on-die including circuitry required for operation ofe.g., CDR 304 as well as circuitry configured for design for test and/ordesign for manufacturing. The eye width measurements may be based on aPRBS bit stream or may be performed using an unknown bit stream, e.g.,actual data.

FIGS. 4A and 4B illustrate examples of waveforms generated during eyemargin characterizations performed using a method consistent with thepresent disclosure. The left eye margin 208 may be characterized asillustrated FIG. 4A in plot 402. A received bit stream is illustrated bya simplified eye margin waveform 404. A recovered clock signalcorresponding to the received clock signal is illustrated by waveform406. A margining clock signal is illustrated by waveform 408. The righteye margin 206 may be characterized as illustrated FIG. 4B in plot 422.A received bit stream is illustrated by a simplified eye margin waveform424. A recovered clock signal corresponding to the received clock signalis illustrated by waveform 426. A margining clock signal is illustratedby waveform 428. At each start of test marker 410, 430, the respectiverecovered clock signal 406, 426 and the respective margining clocksignal 408, 428 are in phase. As time goes on and the marginingcharacterizations proceed, as described herein, the magnitude of thephase differences between respective recovered clock signal 406, 426 andthe respective margining clock signal 408, 428 increase until an erroris detected 412, 432.

A left eye margin 414 and a right eye margin 434 may then be determinedbased, at least in part, on a count of clock cycles (e.g. unitintervals) from the start of test marker 410, 430 to the first bit error412, 432 and the phase difference increment ΔT (e.g., eye margin=numberof clock cycles*phase difference increment). In some embodiments, thecount of unit intervals may be adjusted for a known number oftransitions during the test time period. For example, when the bitstream is a known PRBS, the adjustment may correspond to a number ofadjacent intervals that did not have a bit transition between them. Aneye margin characterization sequence, as described herein, may berepeated a number of times and statistical averages may be obtained forthe measured eye width.

In some embodiments, each characterization sequence may include amaximum allowed time to detect an error and/or a maximum allowed time torecapture (e.g., between sequential tests). Both parameters areconfigured to provide a limit to a testing trial if, for example, thereceived bit stream does not have transitions or after acharacterization sequence has completed, the margining input removed andthe loop closed, the CDR is unable to recapture symbol timing. Suchlimits may be useful for limiting total test time in a productionenvironment.

For example, for 0.5 picosecond resolution, a margining input of 5000PPM and a 10 GB/second data rate, a duration of a full eye width scanmay be 100 picoseconds (50 picoseconds left and 50 picoseconds right). Ascan time may then equal 100 picoseconds divided by 0.5 picoseconds,e.g., a 20 nanosecond scan time to sweep the full eye width. Assuming aduration of a CDR re-lock of about 50 nanoseconds when the receiver isclose to convergence, a likely maximum time for detecting an error andthen re-synching would about 70 nanoseconds. If the number of trialsdesired is on the order of about 10⁷ to about 10⁸, a duration of theentire test including the number of trials corresponds to about 7seconds (70 nsec×10⁸). Typically, the duration of a single scan will beless than 20 nanoseconds, so 7 seconds will seldom be reached. Forexample, the parameters affecting the scan time are open-loop eye-scanphase time and closed loop re-train phase time (which may be impacted byCDR bandwidth).

In some embodiments, a system and method consistent with the presentdisclosure may be utilized to emulate a specific phase shift and to thendwell at that phase shift until a phase noise exceeds a threshold. Phasenoise is typically proportional to a square root of dwell time. Thespecific phase shift may be achieved by applying the margining input asdescribed herein and counting a number of unit intervals that correspondto the specific phase shift divided by the phase difference increment.Once the desired phase shift is achieved, the margining input may beremoved and the system may be configured to dwell at the specific phaseshift for M clock cycles. During the dwell time, the eye pattern may becharacterized as described herein. Thus, effects of phase noise may beinvestigated using a method and system consistent with the presentdisclosure.

FIG. 5A illustrates one example of an eye measurement system 500configured to characterize the eye margin, as described herein. In thisexample, a known PRBS (pseudorandom binary sequence) corresponds to thebit stream in at the input to the transmitter 102. The PRBS may also begenerated in the receiver by PRBS generator 502 and synchronized to thereceived PRBS from the transmitter. Continuing with this example, thereceived PRBS is equalized and amplified by, e.g., equalizer 110, VGA112 and DEF 114 of FIG. 1, and the result is provided to a data slicermodule 504. After the recovered clock signal (output from oscillator512) is synchronized to a receiver clock signal included in the receiverbit stream (output from slicer module 504, e.g., from data slicer 522),a loop filter 506 is decoupled from phase detector and symbol timingmodule 508. A margining input PPM 510 may then be applied to the outputof the loop filter 506, as described herein. As a result, a frequencyoutput of VCO/DCO 512 (margining clock signal) is configured to sweepthe eye width, as described herein.

An output of monitor slicer 520 (monitor bit stream) or data slicer 522(receiver bit stream) may be provided to compare module 526 along withan output of PRBS sequence generator 502 (PRBS bit stream). The counterlogic module 524 may be configured to count UIs (unit intervals, e.g.,clock cycles) based on the PRBS bit stream from a start measurementcommand to at least an error being detected. A first error may bedetected by compare module 526 when the monitor bit stream and/orreceiver bit stream and the PRBS bit stream do not agree. The receiverbit stream may be utilized with the PRBS bit stream to characterize theeye width near zero. Amplitudes of the eye opening may be characterizedusing monitor bit stream and the PRBS bit stream. An effect of anon-zero Vmon is to shift by Vmon a threshold where the output of themonitor slicer 520 transitions from high to low or low to high. Bysweeping the output of VCO/DCO 512 with Vmon nonzero, providescharacterization of the eye width shifted up by +Vmon or down by −Vmonmay be provided. In this manner, the entire eye diagram may becharacterized.

FIG. 5B illustrates another example of an eye measurement system 500′configured to characterize the eye margin, as described herein. Thisexample is similar to test circuit 500 except that rather than thereceiver bit stream being fed back to DFE 540 (corresponding to DFE 114of FIG. 1), the PRBS bit stream generated by the receiver PRBS generator502 is provided to the DFE 540. In this example, by feeding back thereceiver PRBS bit stream, rather than the receiver bit stream, even if asystem has sampling errors, such errors will not propagate through theDFE and close the eye and/or create error multiplication. By avoidingerror multiplication, an eye characterization test may extend beyond afirst error. Thus, the characterization test time may be shortened, morestatistical data may be harvested and error probability distributionsmay be generated, as desired. Thus, more information may be gathered andlearned related to a system's timing jitter and/or behavior. After theeye characterization test is performed, statistical methods may be usedfor, e.g., further analysis, eye-width probability distributiongeneration, eye margining and/or a relation between eye margin and DPM(Device Failures per Million parts shipped).

FIG. 6 illustrates another example of an eye measurement system 600configured to characterize the eye margin as described herein. In thisexample, the transmitted bit stream is not known to be a PRBS sequenceand may be arbitrary, e.g., may include actual data. In this example,errors may be detected by comparing a monitor bit stream to a receiverbit stream when the monitor slicer's reference input Vmon is setslightly higher or lower than the data slicer's reference. Thecomparison may be performed by compare module 606. If the receiver bitstream and monitor bit stream agree, then no error may be detected. Ifthe receiver bit stream and monitor bit stream do not agree, an errormay be detected and an edge of the eye is reached. Thus, an eye diagrammay be characterized using a data sequence that is not known a priori.By adjusting Vmon and repeating an eye width characterization, asdescribed herein, an eye width may be determined at any threshold set byVmon. In other words, when monitor bit stream and the receiver bitstream disagree an output of counter logic module 608 multiplied by thephase difference increment may correspond to eye width at amplitudeVmon. In some embodiments, the eye width characterization may berepeated several times and the results averaged. Such averaging isconfigured to reduce the dependence of the result on the particular datasequence received and to reduce effects on the eye widthcharacterization due to noise and ill-statistics of the data sequence.In this manner, the eye width may be characterized using an arbitraryinput.

FIG. 7 illustrates another embodiment of a system 700 for eye widthmeasurements and margining consistent with the present disclosure.Rather than a VCO or DCO based CDR, CDR module 702 corresponds to aphase interpolator (PI) based CDR module. CDR 702 includes a frequencyand timing synchronization detectors and control loops module 704, aphase locked loop 706 configured to provide frequency synchronizationwith a received signal and a phase interpolator 708 configured toprovide timing synchronization. The frequency and timing synchronizationdetectors and control loops module 704 and PLL 706 are configured tolock reference sine and cosine components (e.g., in-phase and quadrature(I/Q)) to the received clock and data signal (e.g., received bit stream)frequency. The reference I/Q is configured to drive phase interpolator708. Once frequency lock (e.g., recovered clock frequency corresponds tothe receiver clock frequency) is obtained and frequency trackingachieved, the phase no longer drifts across the eye and the samplingclock reaches a nearly constant phase shift from approximately a middleof the eye. A second loop included in frequency and timingsynchronization detectors and control loops module 704 and operatingusing a phase detector or symbol timing detector may lock to the correctsampling phase by modulating the phase interpolator 708 control signals(e.g., shifting phase up or down until proper alignment of the samplingclock to the middle of the eye diagram is obtained).

For example, to implement eye width measurement and/or eye diagramcharacterization using CDR 702, the phase loop should be decoupled at,e.g., point 720, configured to “freeze” the phase loop when enteringeye-scan mode. A constant margining input PPM 710 may then be added tothe frequency loop so PLL 706 now tracks F(ref)+delta (where Fref isobtained by a frequency detector on the incoming signal).

In some embodiments, a PI-based CDR may operate using a referencecrystal instead of locking onto the frequency of the incoming data. Inthese embodiments, the phase loop is configured to add or subtract phaseat a certain rate in order to track a residual frequency offset betweenthe reference and the incoming signal frequency. In order to perform eyescan in these embodiments, rather than freezing the frequency loop, anamount of phase added by the PI phase loop may be adjusted. Suchadjusting is configured to introduce some of the frequency differencebetween the sampling clock and incoming data signal. The phase detectorof the phase loop may also be frozen while the eye width measurement isperformed which will have the effect of eye scan.

For example, if the Fref frequency deviates by ΔF from the datafrequency, the phase loop is configured to add mod(2πΔFt, 2π) phase tomaintain frequency lock to the incoming data. If the added phase isreduced at each given time, a different frequency (PPM) may appear andeffectively scan the eye. Thus, a method and system consistent with thepresent disclosure may be utilized in both an oscillator-based (e.g.,VCO/DCO-based) CDRs as well as PI-based CDRs, as described herein.

FIG. 8 illustrates a flowchart 800 of exemplary operations consistentwith an embodiment of the present disclosure. The operations may beperformed, for example, by any of example apparatuses 500, 500′, 600. Inparticular, flowchart 800 depicts exemplary operations configured tocharacterize an eye width and/or an eye diagram.

Operations of flow chart 800 may begin 802 with initiating a testsequence. For example, transmitter 102 and receiver 106 may be coupled,powered up and a known PRBS sequence or another sequence may be appliedto transmitter 102. The communication system 100 may then be allowed tostabilize. Operation 804 may include synchronizing the recovered clocksignal to the receiver clock signal and achieving a target BER. Thereceiver clock signal may be included in a receiver bit stream. A CDR,e.g., CDR 304 may be configured to synchronize a recovered clock signalto the receiver clock signal based, at least in part, on the receiverbit stream. In some embodiments, Vmon may be adjusted at operation 806.Vmon may be adjusted to shift an eye width measurement up or down, e.g.,to characterize the eye diagram and/or to characterize eye width for anunknown bit stream.

Operation 808 includes opening a CDR loop. For example, phase detectorand symbol timing module may be decoupled from a loop filter atoperation 808. At least some loop filter parameters may be frozen attheir values just prior to opening the loop. An oscillator coupled tothe loop filter may then be configured to oscillate at a same frequencyas just prior to opening the loop, e.g., the margining clock frequencyis synchronized to the receiver clock frequency. A margining input maybe applied to the loop filter at operation 810. The margining input isconfigured to shift the margining clock frequency to provide a frequencydelta between a receiver clock signal frequency and the margining clockfrequency related to a phase difference increment. A counter may beinitiated, counting may be begin and may continue at least until anerror has been detected at operation 812. An eye width for a currenttrial may be determined at operation 814.

Whether the test is done may be determined at operation 820. Forexample, whether the test is done may be determined based on a number oftrials. In another example, whether the test is done may be determinedbased on a duration of the test. If the test is done, an eye width maybe determined at operation 822. For example, the eye width may bedetermined by averaging the eye widths determined for each trial. If thetest is not done, the CDR loop may be closed and the margining inputremoved at operation 824. Program flow may then proceed to operation 804synchronizing the recovered clock signal to the receiver clock signaland achieving a target BER.

Thus, an eye width may be determined and/or an eye diagram may becharacterized by allowing a margining clock signal to synchronize with areceiver clock signal and a BER to be achieved then opening the loop ina CDR and injecting in margining input configured to sweep the eyewidth. A number of trials may be performed and the resulting eye widthmeasurements may be averaged, for example, to determine the eye width.The margining input may be selected to provide a desired resolution inthe eye width measurement.

While FIG. 8 illustrates various operations according one embodiment, itis to be understood that not all of the operations depicted in FIG. 8are necessary for other embodiments. Indeed, it is fully contemplatedherein that in other embodiments of the present disclosure, theoperations depicted in FIG. 8 and/or other operations described hereinmay be combined in a manner not specifically shown in any of thedrawings, but still fully consistent with the present disclosure. Thus,claims directed to features and/or operations that are not exactly shownin one drawing are deemed within the scope and content of the presentdisclosure.

Any of the operations described herein may be implemented in a systemthat includes one or more storage mediums having stored thereon,individually or in combination, instructions that when executed by oneor more processors perform the methods. Here, the processor may include,for example, a CPU, a logic state machine, a sequencer and/or otherprogrammable circuitry. Also, it is intended that operations describedherein may be distributed across a plurality of physical devices, suchas processing structures at more than one different physical locations.The storage medium may include any type of tangible medium, for example,any type of disk including hard disks, floppy disks, optical disks,compact disk read-only memories (CD-ROMs), compact disk rewritables(CD-RWs), and magneto-optical disks, semiconductor devices such asread-only memories (ROMs), random access memories (RAMs) such as dynamicand static RAMs, erasable programmable read-only memories (EPROMs),electrically erasable programmable read-only memories (EEPROMs), flashmemories, Solid State Disks (SSDs), magnetic or optical cards, or anytype of media suitable for storing electronic instructions. Otherembodiments may be implemented as software modules executed by aprogrammable control device. The storage medium may be non-transitory.

As used in any embodiment herein, the term “module” may refer to an app,software, firmware and/or circuitry configured to perform any of theaforementioned operations. Software may be embodied as a softwarepackage, code, instructions, instruction sets and/or data recorded onnon-transitory computer readable storage medium. Firmware may beembodied as code, instructions or instruction sets and/or data that arehard-coded (e.g., nonvolatile) in memory devices.

“Circuitry”, as used in any embodiment herein, may comprise, forexample, singly or in any combination, hardwired circuitry, programmablecircuitry such as computer processors comprising one or more individualinstruction processing cores, state machine circuitry, and/or firmwarethat stores instructions executed by programmable circuitry. The modulesmay, collectively or individually, be embodied as circuitry that formspart of a larger system, for example, an integrated circuit (IC), systemon-chip (SoC), desktop computers, laptop computers, tablet computers,servers, smart phones, etc.

Thus, a system and method consistent with the present disclosure areconfigured to provide eye width measurement and/or eye widthcharacterization for a communication system. The measurement and/orcharacterization may be done on chip, using existing design for testfunctionality. A system and method are configured to provide fineresolution based on a data rate of a bit stream and the margining input.The system and method may be used for receiver clock and data recovery(CDR) circuits that employ voltage controlled oscillators (VCOs) and/ordigitally controlled oscillators (DCOs). The system and method may befurther applied to phase interpolation (PI) based CDR circuits.

According to one aspect there is provided an apparatus. The apparatusmay include a controller module configured to decouple a phase detectorfrom a loop filter of a receiver under test in response to synchronizinga margining clock signal to a receiver clock signal at a receiver clocksignal frequency, the margining clock signal generated by an oscillatorcoupled to the loop filter; a margining input module configured to applya margining input to the loop filter, the margining input configured toshift a frequency of the margining clock signal by a constant amountrelated to the margining input; a compare module configured to compare afirst bit stream and a second bit stream, the comparing configured todetect an error, the first bit stream related to a transmitted bitstream received by the receiver under test; and a counter logic moduleconfigured to count cycles of the receiver clock signal or the marginingclock signal, wherein an eye width associated with the receiver undertest is related to the margining input, the frequency of the receiverclock signal and a count of clock cycles when an error is detected.

Another example apparatus includes the forgoing components and furtherincludes a pseudo-random bit sequence (PRBS) generator configured togenerate a pseudo-random bit sequence synchronized to the marginingclock signal.

Another example apparatus includes the forgoing components and the firstbit stream corresponds to a receiver bit stream and the second bitstream corresponds to the pseudo-random bit sequence.

Another example apparatus includes the forgoing components and the firstbit stream is a receiver bit stream and the second bit stream is relatedto the first bit stream.

Another example apparatus includes the forgoing components and themargining input corresponds to a voltage and the oscillator is a voltagecontrolled oscillator.

Another example apparatus includes the forgoing components and themargining input corresponds to a digital value and the oscillator is adigital controlled oscillator.

According to another aspect there is provided a method. The method mayinclude decoupling a phase detector from a loop filter of a receiverunder test in response to synchronizing a margining clock signal to areceiver clock signal at a receiver clock signal frequency; applying amargining input to the loop filter, the margining input configured toshift a frequency of the margining clock signal by a constant amountrelated to the margining input; comparing a first bit stream and asecond bit stream, the first bit stream related to a transmitted bitstream received by the receiver under test, the comparing configured todetect an error; and counting cycles of the receiver clock signal or themargining clock signal, wherein an eye width associated with thereceiver under test is related to the margining input, the frequency ofthe receiver clock signal and a count of clock cycles when the error isdetected.

Another example method includes the forgoing operations and furtherincludes generating a pseudo-random bit sequence synchronized to themargining clock signal.

Another example method includes the forgoing operations and furtherincludes generating the first bit stream based, at least in part, on themargining clock signal.

Another example method includes the forgoing operations and furtherincludes removing the margining input from the loop filter; recouplingthe phase detector and the loop filter; delaying for a time intervalconfigured to allow the margining clock signal and the receiver clocksignal to synchronize; and halting the test if the margining clocksignal and the receiver clock signal are not synchronized at the end ofthe time interval.

Another example method includes the forgoing operations and furtherincludes removing the margining input from the loop filter after a timeinterval, the time interval corresponding to a phase shift between thereceiver clock signal and the margining clock signal; and dwelling atthe phase shift for a number M of clock cycles while comparing andcounting.

Another example method includes the forgoing operations and the firstbit stream is a receiver bit stream and the second bit stream is relatedto the first bit stream.

Another example method includes the forgoing operations and the firstbit stream corresponds to a receiver bit stream and the second bitstream corresponds to the pseudo-random bit sequence.

According to one aspect there is provided a receiver. The receiver mayinclude a clock and data recovery (CDR) module comprising a phasedetector, a loop filter coupled to an output of the phase detector, andan oscillator coupled to an output of the loop filter, the oscillatorconfigured to generate a margining clock signal; a data slicer moduleconfigured to receive the margining clock signal from the CDR module, togenerate a first bit stream based, at least in part, on the marginingclock signal and to provide a receiver clock signal to the CDR module,the first bit stream related to a transmitted bit stream received by thereceiver; and eye measurement circuitry. The eye measurement circuitrymay include a controller module configured to decouple the phasedetector from the loop filter in response to synchronizing the marginingclock signal with the receiver clock signal at a receiver clock signalfrequency; a margining input module configured to apply a margininginput to the loop filter, the margining input configured to shift afrequency of the margining clock signal by a constant amount related tothe margining input; a compare module configured to compare the firstbit stream and a second bit stream, the comparing configured to detectan error; and a counter logic module configured to count cycles of thereceiver clock signal or the margining clock signal, wherein an eyewidth associated with the receiver under test is related to themargining input, the frequency of the receiver clock signal and a countof clock cycles when the error is detected.

Another example receiver includes the forgoing components and the dataslicer module comprises a data slicer configured to provide a receiverbit stream.

Another example receiver includes the forgoing components and the dataslicer module comprises a monitor slicer configured to provide a monitorbit stream, the monitor bit stream related to a monitor slicer referencevoltage configured to facilitate measuring the eye width at an amplitudecorresponding to the monitor slicer reference voltage.

Another example receiver includes the forgoing components and the dataslicer module comprises a monitor slicer configured to provide thesecond bit stream, the second bit stream related to a monitor slicerreference voltage.

Another example receiver includes the forgoing components and the eyemeasurement circuitry further comprises a pseudo-random bit sequence(PRBS) generator configured to generate a pseudo-random bit sequencesynchronized to the margining clock signal.

Another example receiver includes the forgoing components and furtherincludes a decision feedback equalizer coupled to the data slicer moduleand configured to receive the pseudo-random bit sequence.

Another example receiver includes the forgoing components and theoscillator is a voltage controlled oscillator or a digital controlledoscillator.

According to one aspect there is provided an apparatus. The apparatusmay include a controller module configured to decouple a phase detectorfrom a loop filter of a receiver under test in response to synchronizinga margining clock signal to a receiver clock signal at a receiver clocksignal frequency, the margining clock signal generated by an oscillatorcoupled to the loop filter; a margining input module configured to applya margining input to the loop filter, the margining input configured toshift a frequency of the margining clock signal by a constant amountrelated to the margining input; a compare module configured to compare afirst bit stream and a second bit stream, the comparing configured todetect an error, the first bit stream related to a transmitted bitstream received by the receiver under test; and a counter logic moduleconfigured to count cycles of the receiver clock signal or the marginingclock signal, wherein an eye width associated with the receiver undertest is related to the margining input, the frequency of the receiverclock signal and a count of clock cycles when an error is detected.

Another example apparatus includes the forgoing components and furtherincludes a pseudo-random bit sequence (PRBS) generator configured togenerate a pseudo-random bit sequence synchronized to the marginingclock signal.

Another example apparatus includes the forgoing components and the firstbit stream corresponds to a receiver bit stream and the second bitstream corresponds to the pseudo-random bit sequence.

Another example apparatus includes the forgoing components and the firstbit stream is a receiver bit stream and the second bit stream is relatedto the first bit stream.

Another example apparatus includes the forgoing components and themargining input corresponds to a voltage and the oscillator is a voltagecontrolled oscillator.

Another example apparatus includes the forgoing components and themargining input corresponds to a digital value and the oscillator is adigital controlled oscillator.

According to another aspect there is provided a method. The method mayinclude decoupling a phase detector from a loop filter of a receiverunder test in response to synchronizing a margining clock signal to areceiver clock signal at a receiver clock signal frequency; applying amargining input to the loop filter, the margining input configured toshift a frequency of the margining clock signal by a constant amountrelated to the margining input; comparing a first bit stream and asecond bit stream, the first bit stream related to a transmitted bitstream received by the receiver under test, the comparing configured todetect an error; and counting cycles of the receiver clock signal or themargining clock signal, wherein an eye width associated with thereceiver under test is related to the margining input, the frequency ofthe receiver clock signal and a count of clock cycles when the error isdetected.

Another example method includes the forgoing operations and furtherincludes generating a pseudo-random bit sequence synchronized to themargining clock signal.

Another example method includes the forgoing operations and furtherincludes generating the first bit stream based, at least in part, on themargining clock signal.

Another example method includes the forgoing operations and furtherincludes removing the margining input from the loop filter; recouplingthe phase detector and the loop filter; delaying for a time intervalconfigured to allow the margining clock signal and the receiver clocksignal to synchronize; and halting the test if the margining clocksignal and the receiver clock signal are not synchronized at the end ofthe time interval.

Another example method includes the forgoing operations and furtherincludes removing the margining input from the loop filter after a timeinterval, the time interval corresponding to a phase shift between thereceiver clock signal and the margining clock signal; and dwelling atthe phase shift for a number M of clock cycles while comparing andcounting.

Another example method includes the forgoing operations and the firstbit stream is a receiver bit stream and the second bit stream is relatedto the first bit stream.

Another example method includes the forgoing operations and the firstbit stream corresponds to a receiver bit stream and the second bitstream corresponds to the pseudo-random bit sequence.

According to one aspect there is provided a receiver. The receiver mayinclude a clock and data recovery (CDR) module comprising a phasedetector, a loop filter coupled to an output of the phase detector, andan oscillator coupled to an output of the loop filter, the oscillatorconfigured to generate a margining clock signal; a data slicer moduleconfigured to receive the margining clock signal from the CDR module, togenerate a first bit stream based, at least in part, on the marginingclock signal and to provide a receiver clock signal to the CDR module,the first bit stream related to a transmitted bit stream received by thereceiver; and eye measurement circuitry. The eye measurement circuitrymay include a controller module configured to decouple the phasedetector from the loop filter in response to synchronizing the marginingclock signal with the receiver clock signal at a receiver clock signalfrequency; a margining input module configured to apply a margininginput to the loop filter, the margining input configured to shift afrequency of the margining clock signal by a constant amount related tothe margining input; a compare module configured to compare the firstbit stream and a second bit stream, the comparing configured to detectan error; and a counter logic module configured to count cycles of thereceiver clock signal or the margining clock signal, wherein an eyewidth associated with the receiver under test is related to themargining input, the frequency of the receiver clock signal and a countof clock cycles when the error is detected.

Another example receiver includes the forgoing components and the dataslicer module comprises a data slicer configured to provide a receiverbit stream.

Another example receiver includes the forgoing components and the dataslicer module comprises a monitor slicer configured to provide a monitorbit stream, the monitor bit stream related to a monitor slicer referencevoltage configured to facilitate measuring the eye width at an amplitudecorresponding to the monitor slicer reference voltage.

Another example receiver includes the forgoing components and the dataslicer module comprises a monitor slicer configured to provide thesecond bit stream, the second bit stream related to a monitor slicerreference voltage.

Another example receiver includes the forgoing components and the eyemeasurement circuitry further comprises a pseudo-random bit sequence(PRBS) generator configured to generate a pseudo-random bit sequencesynchronized to the margining clock signal.

Another example receiver includes the forgoing components and furtherincludes a decision feedback equalizer coupled to the data slicer moduleand configured to receive the pseudo-random bit sequence.

Another example receiver includes the forgoing components and theoscillator is a voltage controlled oscillator or a digital controlledoscillator.

According to one aspect there is provided an apparatus. The apparatusmay include a controller module configured to decouple a phase detectorfrom a loop filter of a receiver under test in response to synchronizinga margining clock signal to a receiver clock signal at a receiver clocksignal frequency, the margining clock signal generated by an oscillatorcoupled to the loop filter; a margining input module configured to applya margining input to the loop filter, the margining input configured toshift a frequency of the margining clock signal by a constant amountrelated to the margining input; a compare module configured to compare afirst bit stream and a second bit stream, the comparing configured todetect an error, the first bit stream related to a transmitted bitstream received by the receiver under test; and a counter logic moduleconfigured to count cycles of the receiver clock signal or the marginingclock signal, wherein an eye width associated with the receiver undertest is related to the margining input, the frequency of the receiverclock signal and a count of clock cycles when an error is detected.

Another example apparatus includes the forgoing components and furtherincludes a pseudo-random bit sequence (PRBS) generator configured togenerate a pseudo-random bit sequence synchronized to the marginingclock signal.

Another example apparatus includes the forgoing components and the firstbit stream corresponds to a receiver bit stream the second bit streamcorresponds to the pseudo-random bit sequence.

Another example apparatus includes the forgoing components and the firstbit stream is a receiver bit stream and the second bit stream is relatedto the first bit stream.

Another example apparatus includes the forgoing components and themargining input corresponds to a voltage and the oscillator is a voltagecontrolled oscillator.

Another example apparatus includes the forgoing components and themargining input corresponds to a digital value and the oscillator is adigital controlled oscillator.

According to another aspect there is provided a method. The method mayinclude decoupling a phase detector from a loop filter of a receiverunder test in response to synchronizing a margining clock signal to areceiver clock signal at a receiver clock signal frequency; applying amargining input to the loop filter, the margining input configured toshift a frequency of the margining clock signal by a constant amountrelated to the margining input; comparing a first bit stream and asecond bit stream, the first bit stream related to a transmitted bitstream received by the receiver under test, the comparing configured todetect an error; and counting cycles of the receiver clock signal or themargining clock signal, wherein an eye width associated with thereceiver under test is related to the margining input, the frequency ofthe receiver clock signal and a count of clock cycles when the error isdetected.

Another example method includes the forgoing operations and furtherincludes generating a pseudo-random bit sequence synchronized to themargining clock signal.

Another example method includes the forgoing operations and the firstbit stream is a receiver bit stream and the second bit stream is relatedto the first bit stream.

Another example method includes the forgoing operations and the firstbit stream corresponds to a receiver bit stream and the second bitstream corresponds to the pseudo-random bit sequence.

Another example method includes the forgoing operations and furtherincludes generating the first bit stream based, at least in part, on themargining clock signal.

Another example method includes the forgoing operations and furtherincludes removing the margining input from the loop filter; recouplingthe phase detector and the loop filter; delaying for a time intervalconfigured to allow the margining clock signal and the receiver clocksignal to synchronize; and halting the test if the margining clocksignal and the receiver clock signal are not synchronized at the end ofthe time interval.

Another example method includes the forgoing operations and furtherincludes removing the margining input from the loop filter after a timeinterval, the time interval corresponding to a phase shift between thereceiver clock signal and the margining clock signal; and dwelling atthe phase shift for a number M of clock cycles while comparing andcounting.

According to one aspect there is provided a receiver. The receiver mayinclude a clock and data recovery (CDR) module comprising a phasedetector, a loop filter coupled to an output of the phase detector, andan oscillator coupled to an output of the loop filter, the oscillatorconfigured to generate a margining clock signal; a data slicer moduleconfigured to receive the margining clock signal from the CDR module, togenerate a first bit stream based, at least in part, on the marginingclock signal and to provide a receiver clock signal to the CDR module,the first bit stream related to a transmitted bit stream received by thereceiver; and eye measurement circuitry. The eye measurement circuitrymay include a controller module configured to decouple the phasedetector from the loop filter in response to synchronizing the marginingclock signal with the receiver clock signal at a receiver clock signalfrequency; a margining input module configured to apply a margininginput to the loop filter, the margining input configured to shift afrequency of the margining clock signal by a constant amount related tothe margining input; a compare module configured to compare the firstbit stream and a second bit stream, the comparing configured to detectan error; and a counter logic module configured to count cycles of thereceiver clock signal or the margining clock signal, wherein an eyewidth associated with the receiver under test is related to themargining input, the frequency of the receiver clock signal and a countof clock cycles when the error is detected.

Another example receiver includes the forgoing components and the dataslicer module comprises a data slicer configured to provide a receiverbit stream.

Another example receiver includes the forgoing components and the dataslicer module comprises a monitor slicer configured to provide thesecond bit stream, the second bit stream related to a monitor slicerreference voltage.

Another example receiver includes the forgoing components and the dataslicer module comprises a monitor slicer configured to provide a monitorbit stream, the monitor bit stream related to a monitor slicer referencevoltage configured to facilitate measuring the eye width at an amplitudecorresponding to the monitor slicer reference voltage.

Another example receiver includes the forgoing components and the eyemeasurement circuitry further comprises a pseudo-random bit sequence(PRBS) generator configured to generate a pseudo-random bit sequencesynchronized to the margining clock signal and further comprising adecision feedback equalizer coupled to the data slicer module andconfigured to receive the pseudo-random bit sequence.

Another example receiver includes the forgoing components and theoscillator is a voltage controlled oscillator or a digital controlledoscillator.

According to one aspect there is provided an apparatus. The apparatusmay include a controller module configured to decouple a phase detectorfrom a loop filter of a receiver under test in response to synchronizinga margining clock signal to a receiver clock signal at a receiver clocksignal frequency, the margining clock signal generated by an oscillatorcoupled to the loop filter; a margining input module configured to applya margining input to the loop filter, the margining input configured toshift a frequency of the margining clock signal by a constant amountrelated to the margining input; a compare module configured to compare afirst bit stream and a second bit stream, the comparing configured todetect an error, the first bit stream related to a transmitted bitstream received by the receiver under test; and a counter logic moduleconfigured to count cycles of the receiver clock signal or the marginingclock signal, wherein an eye width associated with the receiver undertest is related to the margining input, the frequency of the receiverclock signal and a count of clock cycles when an error is detected.

Another example apparatus includes the forgoing components and furtherincludes a pseudo-random bit sequence (PRBS) generator configured togenerate a pseudo-random bit sequence synchronized to the marginingclock signal wherein the first bit stream corresponds to a receiver bitstream and the second bit stream corresponds to the pseudo-random bitsequence.

Another example apparatus includes the forgoing components and the firstbit stream is a receiver bit stream and the second bit stream is relatedto the first bit stream.

Another example apparatus includes the forgoing components and themargining input corresponds to a voltage and the oscillator is a voltagecontrolled oscillator or the margining input corresponds to a digitalvalue and the oscillator is a digital controlled oscillator.

According to another aspect there is provided a method. The method mayinclude decoupling a phase detector from a loop filter of a receiverunder test in response to synchronizing a margining clock signal to areceiver clock signal at a receiver clock signal frequency; applying amargining input to the loop filter, the margining input configured toshift a frequency of the margining clock signal by a constant amountrelated to the margining input; comparing a first bit stream and asecond bit stream, the first bit stream related to a transmitted bitstream received by the receiver under test, the comparing configured todetect an error; and counting cycles of the receiver clock signal or themargining clock signal, wherein an eye width associated with thereceiver under test is related to the margining input, the frequency ofthe receiver clock signal and a count of clock cycles when the error isdetected.

Another example method includes the forgoing operations and furtherincludes generating a pseudo-random bit sequence synchronized to themargining clock signal, wherein the first bit stream corresponds to areceiver bit stream and the second bit stream corresponds to thepseudo-random bit sequence.

Another example method includes the forgoing operations and the firstbit stream is a receiver bit stream and the second bit stream is relatedto the first bit stream.

Another example method includes the forgoing operations and furtherincludes generating the first bit stream based, at least in part, on themargining clock signal.

Another example method includes the forgoing operations and furtherincludes removing the margining input from the loop filter; recouplingthe phase detector and the loop filter; delaying for a time intervalconfigured to allow the margining clock signal and the receiver clocksignal to synchronize; and halting the test if the margining clocksignal and the receiver clock signal are not synchronized at the end ofthe time interval.

Another example method includes the forgoing operations and furtherincludes removing the margining input from the loop filter after a timeinterval, the time interval corresponding to a phase shift between thereceiver clock signal and the margining clock signal; and dwelling atthe phase shift for a number M of clock cycles while comparing andcounting.

According to one aspect there is provided a receiver. The receiver mayinclude a clock and data recovery (CDR) module comprising a phasedetector, a loop filter coupled to an output of the phase detector, andan oscillator coupled to an output of the loop filter, the oscillatorconfigured to generate a margining clock signal; a data slicer moduleconfigured to receive the margining clock signal from the CDR module, togenerate a first bit stream based, at least in part, on the marginingclock signal and to provide a receiver clock signal to the CDR module,the first bit stream related to a transmitted bit stream received by thereceiver; and eye measurement circuitry. The eye measurement circuitrymay include a controller module configured to decouple the phasedetector from the loop filter in response to synchronizing the marginingclock signal with the receiver clock signal at a receiver clock signalfrequency; a margining input module configured to apply a margininginput to the loop filter, the margining input configured to shift afrequency of the margining clock signal by a constant amount related tothe margining input; a compare module configured to compare the firstbit stream and a second bit stream, the comparing configured to detectan error; and a counter logic module configured to count cycles of thereceiver clock signal or the margining clock signal, wherein an eyewidth associated with the receiver under test is related to themargining input, the frequency of the receiver clock signal and a countof clock cycles when the error is detected.

Another example receiver includes the forgoing components and the dataslicer module comprises a data slicer configured to provide a receiverbit stream.

Another example receiver includes the forgoing components and the dataslicer module comprises a monitor slicer configured to provide thesecond bit stream, the second bit stream related to a monitor slicerreference voltage.

Another example receiver includes the forgoing components and the dataslicer module comprises a monitor slicer configured to provide a monitorbit stream, the monitor bit stream related to a monitor slicer referencevoltage configured to facilitate measuring the eye width at an amplitudecorresponding to the monitor slicer reference voltage.

Another example receiver includes the forgoing components and the eyemeasurement circuitry further comprises a pseudo-random bit sequence(PRBS) generator configured to generate a pseudo-random bit sequencesynchronized to the margining clock signal and further comprising adecision feedback equalizer coupled to the data slicer module andconfigured to receive the pseudo-random bit sequence.

The terms and expressions which have been employed herein are used asterms of description and not of limitation, and there is no intention,in the use of such terms and expressions, of excluding any equivalentsof the features shown and described (or portions thereof), and it isrecognized that various modifications are possible within the scope ofthe claims. Accordingly, the claims are intended to cover all suchequivalents.

What is claimed is:
 1. A method for eye measurement comprising:receiving a recovered clock signal, the recovered clock signal beingsynchronized with a receiver clock signal; generating a margining clocksignal, wherein the recovered clock signal and the margining clocksignal are in phase for at least one clock cycle; applying a phasedifference increment to the margining clock signal for a plurality ofclock cycles; comparing the margining clock signal to the recoveredclock signal for each clock cycle; counting clock cycles at least untilan error is detected, the error being detected based upon a result ofthe comparison; and determining an eye width of the receiver clocksignal based, at least in part, on the counted clock cycles when theerror was detected and the phase difference increment.
 2. The method ofclaim 1, wherein the phase difference increment is determined based, atleast in part, on a clock accuracy and the recovered clock signal. 3.The method of claim 2, wherein the clock accuracy is measured in partsper million (PPM).
 4. The method of claim 1, wherein the eye width isdetermined in response to the error being detected.
 5. The method ofclaim 4, wherein the eye width is proportional to a product of thecounted clock cycles and the phase difference increment.
 6. The methodof claim 1, wherein counting clock cycles at least until an error isdetected, the error being detected based upon a result of the comparisoncomprises: resetting a clock cycle count when a first error is detected,the first error being detected based upon a result of the comparison;applying an inverted phase difference increment to the margining clocksignal for a second plurality of clock cycles subsequent to the firsterror being detected; and counting clock cycles at least until a seconderror is detected, the second error being detected based upon a resultof the comparison; and wherein the eye width of the receiver clocksignal is determined based, at least in part, on the counted clockcycles when the second error was detected and the inverted phasedifference increment.
 7. An on-die eye measurement system comprising:clock and data recovery circuitry to generate a recovered clock signal,the recovered clock signal being synchronized with a receiver clocksignal; margining circuitry to: generate a margining clock signal, themargining clock signal being in phase with the recovered clock signalfor at least one clock cycle; and apply a phase difference increment tothe margining clock signal for a plurality of clock cycles; andcomparison circuitry to compare the margining clock signal to therecovered clock signal; counter circuitry to count a number of clockcycles at least until an error is detected, the error being detectedbased, at least in part, upon an output of the comparison circuitry; andeye measurement circuitry to determine an eye width of the receiverclock signal based, at least in part, on the counted clock cycles whenthe error was detected and the phase difference increment.
 8. The on-dieeye measurement system of claim 7, wherein the phase differenceincrement is determined based, at least in part, on a clock accuracy andthe recovered clock signal.
 9. The on-die eye measurement system ofclaim 8, wherein the clock accuracy is measured in parts per million(PPM).
 10. The on-die eye measurement system of claim 7, wherein the eyemeasurement circuitry determines the eye width in response to the errorbeing detected.
 11. The on-die measurement system of claim 10, whereinthe eye width is proportional to a product of the counted clock cyclesand the phase difference increment.
 12. The on-die eye measurementsystem of claim 7, wherein the counter circuitry is reset when an erroris detected.
 13. The on-die eye measurement system of claim 7, wherein,responsive to a first error being detected: the counter circuitry isreset; the margining circuitry is to apply an inverted phase differenceincrement to the margining clock signal for a second plurality of clockcycles; and the counter circuitry is to count a second number of clockcycles until a second error is detected, the second error being detectedbased, at least in part, upon an output of the comparison circuitry; andwherein the eye width measurement circuitry is to determine an eye widthof the receiver clock signal based, at least in part, on the countedclock cycles when the second error was detected and the inverted phasedifference increment.
 14. At least one non-transitory computer-readablestorage medium having instructions stored thereon which when executed bya processor result in the following operations for measuring an eye,comprising: receive a recovered clock signal, the recovered clock signalbeing synchronized with a receiver clock signal; generate a marginingclock signal, wherein the recovered clock signal and the margining clocksignal are in phase for at least one clock cycle; apply a phasedifference increment to the margining clock signal for a plurality ofclock cycles; compare the margining clock signal to the recovered clocksignal for each clock cycle; count clock cycles at least until an erroris detected, the error being detected based upon a result of thecomparison; and determine an eye width of the receiver clock signalbased, at least in part, on the counted clock cycles when the error wasdetected and the phase difference increment.
 15. The non-transitorycomputer-readable storage medium of claim 14, wherein the phasedifference increment is determined based, at least in part, on a clockaccuracy and the recovered clock signal.
 16. The non-transitorycomputer-readable storage medium of claim 15, wherein the clock accuracyis measured in parts per million (PPM).
 17. The non-transitorycomputer-readable storage medium of claim 14, wherein the eye width isdetermined in response to the error being detected.
 18. Thenon-transitory computer-readable storage medium of claim 17, wherein theeye width is proportional to a product of the counted clock cycles andthe phase difference increment.
 19. The non-transitory computer-readablestorage medium of claim 1, wherein the instructions resulting in theoperations count clock cycles at least until an error is detected, theerror being detected based upon a result of the comparison, whenexecuted by the processor, result in further operations comprising:reset a clock cycle count when a first error is detected, the firsterror being detected based upon a result of the comparison; apply aninverted phase difference increment to the margining clock signal for asecond plurality of clock cycles subsequent to the first error beingdetected; and count clock cycles at least until a second error isdetected, the second error being detected based upon a result of thecomparison; and wherein the eye width of the receiver clock signal isdetermined based, at least in part, on the counted clock cycles when thesecond error was detected and the inverted phase difference increment.20. An eye measurement system comprising: a means for receiving arecovered clock signal, the recovered clock signal being synchronizedwith a receiver clock signal; a means for generating a margining clocksignal, wherein the recovered clock signal and the margining clocksignal are in phase for at least one clock cycle; a means for applying aphase difference increment to the margining clock signal for a pluralityof clock cycles; a means for comparing the margining clock signal to therecovered clock signal for each clock cycle; a means for counting clockcycles at least until an error is detected, the error being detectedbased upon a result of the comparison; and a means for determining aneye width of the receiver clock signal based, at least in part, on thecounted clock cycles when the error was detected and the phasedifference increment.
 21. The eye measurement system of claim 20,wherein the phase difference increment is determined based, at least inpart, on a clock accuracy and the recovered clock signal.
 22. The eyemeasurement system of claim 21, wherein the clock accuracy is measuredin parts per million (PPM).
 23. The eye measurement system of claim 20,wherein the eye width is determined in response to the error beingdetected.
 24. The eye measurement system of claim 23, wherein the eyewidth is proportional to a product of the counted clock cycles and thephase difference increment.
 25. The eye measurement system of claim 20,wherein the means for counting clock cycles at least until an error isdetected, the error being detected based upon a result of the comparisoncomprises: a means for resetting a clock cycle count when a first erroris detected, the first error being detected based upon a result of thecomparison; a means for applying an inverted phase difference incrementto the margining clock signal for a second plurality of clock cyclessubsequent to the first error being detected; and a means for countingclock cycles at least until a second error is detected, the second errorbeing detected based upon a result of the comparison; and wherein theeye width of the receiver clock signal is determined based, at least inpart, on the counted clock cycles when the second error was detected andthe inverted phase difference increment.